1. Field of the Invention
This invention relates to an image generating apparatus and, more particularly, to an image generating apparatus used in a video equipment employing a computer, such as a graphics computer, a special effect device or a video game machine.
2. Related Art
In an equipment employing computer graphics, such as a video game machine, a personal computer or a graphics computer, geometry processing or rendering processing is carried out for generating data of a picture or an image outputted to and displayed on a television receiver, a monitor receiver or a cathode ray tube (CRT) display device. A geometry processing unit performs coordinate transformation, clipping or light source calculations, while a rendering processing unit takes account of colors and Z-values of all pixels making up a polygon, from the color data of apex points and Z-values indicating the depth, for writing pixel data in an image memory.
For performing such processing, in particular the above-mentioned rendering processing, a drawing processing circuit or a drawing engine is used. The drawing engine generally employs a method of writing a generated pixel directly in an image memory.
Meanwhile, the image drawing speed is influenced by the speed of writing from the drawing engine to the image memory, such that, with a slow image memory accessing speed, the drawing speed is lowered. Therefore, if an expensive high-speed memory is used as a large-capacity image memory for increasing the drawing speed, the system cost is prohibitively increased. However, if an inexpensive dynamic random access memory (DRAM) is used, the drawing speed of the system is lowered.
It may be envisaged to provide a buffer memory corresponding to a DRAM page between the drawing engine and the image memory, and to make image memory accessing only for high-speed burst transfer. However, if drawing data occurs in a sequence of bridging the page boundary within the range of the buffer memory capacity, the lowered efficiency results.
In view of the foregoing, the present invention provides an image generating apparatus whereby a high drawing speed may be maintained even with the use of an inexpensive memory, such as a DRAM.